Download Visual Measurements of the Multiple Star

Vol. 7 No. 1 January 1, 2011
Journal of Double Star Observations
Page 50
Visual Measurements of the Multiple Star
STT 269 AB-C and ARN 8 AB-D
Thomas G. Frey1, Irina Achildiyev2, Chandra Alduenda2,
Reid Bridgeman2, Rebecca Chamberlain2, and Alex Hendrix2
1. California Polytechnic State University, San Luis Obispo
2. Evergreen State College, Olympia, Washington
Abstract: This astrometry project was performed by a member of the faculty and students
from The Evergreen State College at the 2010 Pine Mountain Observatory Summer Science
Research Workshop. This study involved measuring and analyzing the separation and position angles of the multiple star system STT 269 AB-C and ARN 8 AB-D. The astrometric
binary AB was treated as a single star. Separation and position angles of the C and D components relative to AB were made. Percent differences between observed and literature values were all less than 1 percent.
Introduction
A multiple star astrometry investigation was
performed by an interdisciplinary team (Figure 1)
from The Evergreen State College (TESC) at the
Pine Mountain Observatory (PMO) Summer Science
Research Workshop near Bend, Oregon. The workshop was conducted August 5-8, 2010. This investigation involved the separation and position angle
measurements of the multiple star system, STT 269
AB-C and ARN 8 AB-D, shown in a modified ALADIN image in Figure 2 (ALADIN previewer, RA: 13
32 50.99, Dec: +34 54 25.69).
Equipment
The instrumentation used in this investigation
was the same as has been previously reported [Frey
2008], namely, an 18-inch Newtonian telescope by
Obsession on an alt-az mount. The tracking unit was
StellarCAT’s ServoCAT GOTO system guided by an
Argo Navis computer. A Celestron 12.5 mm Micro
Guide astrometric eyepiece was used to measure the
separation and position angles. Calibration of the
Celestron eyepiece was unnecessary because the telescope was in the same configuration as in a previous
study [Frey, et. al. 2010]. The scale constant for the
linear scale, as previously determined, was 10.24 arc
seconds per division. The standard deviation for the
Figure 1. Clockwise from top right: Thomas Frey, Alex
Hendrix, Rebecca Chamberlain, Irina Achildiyev, Reid Brickman, and Chandra Alduenda. The team is gathered around
Frey’s 18” alt- az Obsession telescope used in this study.
11 observations was 0.94 arc seconds/division; the
standard error of the mean was 0.29 arc seconds/
division.
Locale and Observing Conditions
Pine Mountain is located east of Bend, Oregon at
+43.79 degrees north latitude and 120.94 degrees
Vol. 7 No. 1 January 1, 2011
Journal of Double Star Observations
Page 51
Visual Measurements of the Multiple Star STT 269 AB-C and ARN 8 AB-D
Figure 2: The multiple star system STT 269 AB- C
and ARN 8 AB- D
stars are common, such as !279 in Andromeda (Haas
2006). The three stars are indicated by the letters A,
B, and C listed in this order and, usually, in diminishing brightness. In essence, this study is like measuring two double stars in the same field of view, with
the same primary star. Since triple star measurements are less often reported, the students decided to
take on the challenge.
The multiple star chosen was STT 269 AB-C and
ARN 8 AB-D. Frey had originally requested a list of
triple stars from Brian Mason, Director of the Washington Double Star Catalog (WDS) at the US Naval
Observatory [Mason, 2010]. His request included
stars with no RA limits, declinations limited from
-15 to +90 degrees, separations greater than 15 arc
seconds and magnitudes for each star no greater than
11. Mason graciously sent a list of 26 possible candidates. As an added bonus the 26 were actually quaternary systems. The four-star system selected for observation by the TESC team visually appears as a triple
star since the A and B components have a very small
separation. The stars are in the constellation Canes
Venatici.
west longitude. The sky was clear and the seeing was
excellent with a minimum of scintillation. The temperature during observing hours was 40-45 degrees
Fahrenheit. Breezy conditions existed each night with
occasional strong gusts that did lead to a few outliers
that were recorded, but were deleted from the analy- Historical Information on STT 269 AB
The investigation of STT 269 AB began in 1843
sis.
when it was discovered by Otto Struve, the son of the
Background
Russian astronomer Friedrich G. W. von Struve. In
In past workshops and undergraduate research 1839 the Russian government called upon Friedrich
seminars. double star investigations often follow the to build and direct the new Imperial Observatory at
pattern of (1) calibration of the eyepiece, (2) collecting Pulkowa. The principle instrument was an equatorial
separation and position angle measurements on a refractor with a 15-inch objective lens. This was the
“known “ double star (a system that has been exten- largest refractor in the world at the time just as the
sively studied), and (3) repeating this operation on a 9.5-inch Fraunhofer refractor at Dorpat Observatory
“neglected” double star. Neglected double stars are had been in 1824. It was thought that this 15-inch
systems that have been rarely studied or have not instrument might reveal double stars that had esbeen restudied for quite a while. For investigators caped detection by the Dorpat refractor due to smaller
new to double star research, the neglected double angular separations or faint components [Aitken
stars are chosen to be fairly bright and have separa- 1935]. Otto Struve’s observations of STT 269 AB retions of 25 arc seconds or greater because only an as- vealed a binary system having magnitudes 7.3 and
tronomic eyepiece is being used without a Barlow lens 8.1 with a position angle of 210 degrees and a separaand cannot effectively measure dim stars or ones with tion of 0.3 arc seconds (Mason 2009). This separation
small separations. The choice of neglected double was just within the Dawes limit for this instrument.
stars in the northern hemisphere has diminished in
In 1879 the third star of the system was identified
recent years thanks to the Herculean efforts of such as the “C” component of STT 269 AB-C with a posivisual observers as David Arnold in Flagstaff, Ari- tion angle of 333 degrees relative to the AB compozona. The ones remaining on the neglected double nents, a separation of 116.5 arc seconds, and magnistar list are either too dim or too close together to be tude of the “C” star of 9.2. Its spectral type is F5.
easily evaluated by beginners.
So the Pine Mountain Observatory team decided The Dawes Limit
If two stars of approximately the same magnitude
to measure a visual triple star system instead of atare
close enough that their Airy disks overlap, they
tempting one of the neglected double stars. Triple
Journal of Double Star Observations
Vol. 7 No. 1 January 1, 2011
Page 52
Visual Measurements of the Multiple Star STT 269 AB-C and ARN 8 AB-D
will be seen as an elongated single star. If these disks
generate a figure-8 shape and the light intensity between the two touching disks drops by 30%, the two
stars can still be viewed as separate stars. William
Dawes (1799-1868), an English astronomer, discovered that the smallest separation between two stars
manifesting this 30% drop in intensity can be determined by the empirically developed formula
Dawes Limit = 4.56 arc seconds/A
leaving a total of 17 observations. The separation of
the AB-C system in the current study was determined
to be 123.4 arc seconds. The most recent WDS separation, determined in 2002, was 122.5 arc seconds. The
standard deviation of our observations was 0.53 arc
seconds, while the standard error of the mean was
0.13 arc seconds. The percent difference from the
most recent literature value was 0.74%. These values
are summarized in Table 1. The Besselian epoch for
all measurements recorded in this study was
B2010.597.
The position angle was then evaluated. Some 15
trials with no outliers were observed. Our observationally determined position angle was 331 degrees,
compared to the WDS value of 332 degrees. The standard deviation was 0.66 degrees and the standard
error of the mean was 0.18 degrees.
The percent
difference from the most recent literature value was
-0.30%. These values are summarized in Table 2.
where A is the aperture of the objective lens in inches.
The larger the aperture, the smaller the Dawes limit.
For a 15-inch refractor, the Dawes limit is 0.304 arc
seconds. Dave Arnold (Arnold 2010), a well known
double star observer, pointed out that Struve’s 15inch refractor had the theoretical capability to resolve
this pair, especially since the difference in magnitudes were minimal (7.3 and 8.1). The combined
magnitude of the two stars in STT 269 AB listed in
Separation and Position Angle Measthe WDS is 6.8.
urements of ARN 8 AB-D
Current Separation and Position Angle
of STT 269 AB
In 2007, according to the WDS, the position angle
of STT 269 AB had changed to 220 degrees while the
separation had remained constant at 0.3 arc seconds.
This system, also known as ADS 8939, has been used
as an astrometric standard to calibrate deformable
secondary mirror adaptive optic systems [Close 2003].
Its separation of 0.3 arc seconds in our telescope, using just the Celestron astrometric eyepiece, cannot be
detected and thus it appears as a single star.
Separation and Position Angle Measurements of STT 269 AB-C
After performing two star alignment and engaging the servo motors, 21 separation trials were taken
and 4 outliers were deleted due to strong wind gusts
The separation measurements of both STT 269
AB-C and ARN 8 AB-D were taken consecutively
while the servo motors were still engaged. After the
separation measurements had been completed, the
servo motors were disengaged and the position angles
evaluated by the drift method. Both operations were
easily manipulated because all stars in the system
were in the same field of view.
In 1919, the fourth “D” star was incorporated into
the system. It was originally observed with position
angle of 259 degrees from STT 269 AB, a separation
of 357.1 arc seconds, and the magnitude of the “D”
component was 8.4. Dave Arnold is the official discoverer of ARN 8 AB-D, dated in 2002. The 1919 data
were obtained from other catalogs available to the
Naval Observatory (Mason 2010). Arnold’s discovery
of the AB-D system occurred in 2002 with an 8”
Table 1: Separation Measurements for STT 269 AB- C. *plus 4 Outliers
Star
System
STT 269AB-C
Identifiers
13 32 52.1
+34 54 25.8
Lit.
Epoch
#
Obs.
SD/ME
Obs.
Sep.
Lit.
Sep.
%
Diff.
2002
17*
0.53/0.13
123.4
122.5
0.74
Table 2: Position Angle Measurements for STT 269 AB- C
Star System
STT 269AB-C
Identifiers
13 32 52.1
+34 54 25.8
Lit
Epoch
#
Obs.
SD/ME
Obs.
PA
Lit.
PA
%
Diff.
2002
15
0.66/0.18
331
332
-0.30
Journal of Double Star Observations
Vol. 7 No. 1 January 1, 2011
Page 53
Visual Measurements of the Multiple Star STT 269 AB-C and ARN 8 AB-D
Table 3: Separation Measurements for ARN 8 AB- D. *plus 1 Outlier
Star
System
ARN
Identifiers
8AB-D
13 32 52.1
+34 54 25.8
Lit.
Epoch
#
Obs.
SD/ME
Obs.
Sep.
Lit.
Sep.
% Diff.
2002
19*
0.42/0.10
349.7
352.5
-0.79
Table 4:. Position Angle Measurements for ARN 8 AB- D
Star System
ARN
8AB-D
Identifiers
13 32 52.1
+34 54 25.8
Lit
Epoch
#
Obs.
SD/ME
Obs.
PA
Lit.
PA
% Diff.
2002
12
1.08/0.31
258
260
-0.77
Schmidt Cassegrain telescope. The results were published in 2003 [Arnold 2003].
In 2002, position angle and separation measurements were taken yielding 260 degrees and 352.5 arc
seconds, respectively. In the current study, 20 trials
with 1 outlier deleted resulted in a separation of
349.7 arc seconds, with a standard deviation of 0.42
arc seconds and a standard error of the mean of 0.10
arc seconds. The percent difference from the most recent literature value was -0.79%. This information is
summarized in Table 3.
The position angle was then observed with 12 trials (no outliers). The experimentally determined position angle was 258 degrees, compared to the most recent WDS value of 260 degrees. The standard deviation was 1.08 degrees and the standard error of the
mean was 0.31 degrees. The percent difference from
the most recent literature value was -0.77%. These
values are summarized in Table 4.
Analysis
Triple star systems, analogous to double stars,
can either be bound by gravity or just optically
aligned by coincidence. For instance, the Alpha Centauri system is composed of two sun-like stars, Centauri AB, orbiting one another every 80 years, and
Proximi Centauri located at a distance of about
15,000 AU, orbiting the AB components about every
100,000 to 150,000 years. All three components are
bound by gravity. Beta Cephei, however, is composed
of a true binary star and an optical component. In
some other triple systems none of the components are
bound by gravity due to the great distances between
them, i.e. their alignment is just coincidental.
In order to specify the binary or optical nature of
the star system it is necessary to examine the proper
motion vectors and relative distances between the
components. Some of the physical parameters for the
star system STT 269 AB-C and ARN 8 AB-D are summarized in Table 5. Proper motion vectors and parallax values are given in milli-arc seconds per year
(mas/yr).
Experienced double star observer, Dave Arnold,
[Arnold 2010] explains that in order to be considered
a true binary star, the proper motion vectors of a pair
should have about 90% agreement. Examination of
the AB-C and AB-D proper motion vectors in Table 5
shows that these vectors lay outside these restrictions
and should be considered optical components. However, the STT 269 AB system has been studied at
length [Close 2003] and with a separation of 0.3 arc
seconds is considered an astrometric binary.
Examination of the standard deviation and standard error of the mean for separation and position
angle for this system shows our measurements to be
precise and in good agreement with the Washington
Double Star data of 2002.
Figure 3 shows a modified ALADIN applet photo
Table 5: Parameters for SST 269 AB- C and ARN 8 AB- D
Star
System
Proper
Motion1
(mas/yr)
Parallax
(mas/yr)
Distance
(parsecs)
Sep.1
(arcsec)
Spectral
Type
WDS
Epoch
STT 269AB
(A)-046-015
5.732
174.5
0.3
(A) A6III1
2007
STT 269AB-C
STT 269AB-D
(AB)-046-015
(C) -082+037
(AB)-046-015
(D) +013-027
4.36
3
2.313
1-Washington Double Star Catalog,
3
122.5
(C) F5
432.93
352.9
(D) G03
229.5
2-SIMBAD,
3-The Sky6
3
2002
2002
Vol. 7 No. 1 January 1, 2011
Journal of Double Star Observations
Page 54
Visual Measurements of the Multiple Star STT 269 AB-C and ARN 8 AB-D
Figure 3: Proper motion vectors for STT 269 AB- C,
ARN 8 AB- D.
with enhanced proper motion vectors. Arrows are
shown to approximate scale and direction. The vectors
show pictorially the divergent proper motion vectors
indicating the optical nature of the AB, C, and D
stars.
Conclusions
The separation measurements for both components were precise with standard deviation and mean
error values all below 1.0 arc seconds. The experimental difference in separation for STT 269 AB-C was
less than 1 arc second from the current WDS literature value. However, the experimental value for the
separation of ARN 8 AB-D differed from the literature
value by 2.8 arc seconds. One possibility for this difference could be due to a persistent breeze that was
causing the two stars to jitter around on the linear
scale making precise measurements more difficult.
The larger separation was an additional factor. With
a separation of about 350 arc seconds, and a scale
constant of about 10 arc seconds per division, it required the observer to scan about 35 divisions on the
linear scale, simultaneously, to get the appropriate
separation. Actually, the observer would often cite
one value on the scale and the second value several
seconds later. During this hiatus, the breeze could
easily have moved the telescope so the second value
would be inaccurate.
Future studies of multiple stars will concentrate
on systems with smaller angular separations when
true binary star systems may be investigated.
The position angle measurement for ARN 8 AB-D
also suffered from the wide separation. For STT 269
AB-C, the experimental separation of 123.4 arc seconds was small enough to allow easy alignment on
the linear scale. The position angle was then precisely
measured to within a degree of the literature value
and standard deviation and mean error values less
than 1.0. But the observed separation of ARN 8 AB-D
was over 2.8 times larger than that of STT 269 AB-C.
Team members found it more challenging to align the
two stars on the linear scale before performing the
drift method of position angle determination. The resulting values for the position angle had a wider
spread (standard deviation of 1.08 degrees) and an
experimental value 2 degrees from the literature
value.
Through these experiences, the students gained
confidence and the ability to apply these newly
crafted skills toward further work on the “push-Dobs”
available at The Evergreen State College.
Acknowledgements
The team wants to thank Dave Arnold, Russ
Genet, and Tom Smith for reviewing this paper and
for their suggestions and corrections. A special thanks
is extended to Brian Mason at the U.S. Naval Observatory for additional information on the multiple star
system studied. We also want to thank Gregory Bothun, Director of Pine Mountain Observatory, Mark
Dunaway, Kent Fairfield, Allan Chambers, and Rick
Kang, for opening the facility and being on hand to
answer questions and assist our needs. A special
thanks goes to Danyal Medley at Celestron for the
donation of a 12.5 mm Micro Guide astrometric eyepiece to Evergreen State College, and to Theresa
Aragon (Dean of TESC Summer School), and Peter
Robinson (Director of Lab I and Lab II, and Science
Technician at TESC), for purchasing an additional
12.5 mm Micro Guide astrometric eyepiece. Thanks to
Sarah Pederson, Tina Pearson, Lori Moore, Sharon
Wendt, Frank Barber, Katie Frank, and the other
members of the TESC community for their support.
The workshop would never have been as successful
without the organization, dedication, and tireless effort of Richard Berry as Workshop Director. Our team
also wants to recognize the other double star team
leaders, Jo Johnson and Chris Estrada, for their cooperation and assistance with the project. Finally, if it
Vol. 7 No. 1 January 1, 2011
Journal of Double Star Observations
Page 55
Visual Measurements of the Multiple Star STT 269 AB-C and ARN 8 AB-D
weren’t for Russ Genet’s initial efforts, none of the Close, Laird M., et.al., The Astrophysical Journal,
double star projects would have taken place.
599, 537-547, 2003.
References
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Frey, Thomas G., Spring 2008, Journal of Double Star
Observations, 3(2), p. 59-65.
Frey, Thomas G., et. al., submitted JDSO Sept. 2010.
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http://aladin.u-strasbg.fr/java/alapre.pl.
Haas, Sissy, Double Stars for Small Telescopes, Sky
Publishing, Cambridge, MA, 2006, p.15.
ALADIN database (previewer):
http://aladin.u-strasbg.fr/alapre.pl.
Mason, Brian, The Washington Double Star Catalog.
July 2009. Astrometry Department, U.S. Naval
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Arnold, D., The Double Star Observer, 9(1), 2003.
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